In the field of LVSEM and the related industrial fields which employ the principle of LVSEM to observe features on a specimen surface, such as defect review and defect inspection of wafers or masks for yield management in semiconductor manufacture, forming an image of a specimen surface with high contrast and high resolution for the interested features has been demanded and pursued.
In a LVSEM, a primary electron (PE) beam is generated by an electron source, and then focused onto and forced to scan the examined specimen surface by electron optics. The electron optics generally comprises a condenser lens, an objective lens and a deflector. The PE beam interacts with the specimen and make the specimen release a secondary emission beam therefrom. The secondary emission beam comprises secondary electrons (energy ≦50 eV) and backscattered electrons (50 eV<energy≦PE energy). To limit both the radiation damage on the specimen and the PE beam/specimen interaction to a very small volume beneath the specimen surface, the PE beam is designed to land on the specimen with low energy (<5 keV). In this case, the secondary electrons (SEs) and backscattered electrons (BSEs) are mainly related to the features on the specimen surface where the PE beam lands on, thereby becoming appropriate signal electrons for forming images of the specimen surface. The SEs and/or BSEs are detected by detectors to obtain images of the examined specimen surface.
The contrast of the image in the LVSEM can be generated by many factors and mainly depends on the detection of the signal electrons (SEs and BSEs). The yield δ of the secondary electron (SE) emission (ratio of the number of SEs and the number of primary electrons) changes with the incidence angle (relative to the specimen surface normal) of the PE beam and not sensitive to atomic numbers of the materials of the specimen surface, and therefore a SE image (obtained by detecting SEs) can show the topography of the specimen surface due to geometric inclination differences (topography contrast). The coefficient η of the backscattered electron (BSE) emission (ratio of the number of BSEs and the number of primary electrons) changes with atomic numbers of the materials of the specimen surface and less sensitive to the local inclinations on the specimen surface, and as a result a BSE image (obtained by detecting BSEs) can show material differences (material contrast) of the specimen surface. Furthermore, a Low-Loss (LL) BSE image (obtained by detecting BSEs with energy loss of around 100 eV or less) can exhibit a good contrast for nano-material composition with subtle compositional variation. Besides, an SE image can show potential differences (voltage contrast) on the specimen surface because SE yield δ also changes with PE energy and SEs are too slow to sustain the influence of the additional electric field generated by the potential differences.
Although the resolution of an image in the LVSEM is fundamentally determined by the aberrations of the electron optics thereof, however many factors can deteriorate it. An amount of net electrical charges appears on the specimen surface if the total yield σ(=δ+η) of the secondary emission is not equal to 1. As an SE image is easily influenced by the charges, BSE images have become very important for high resolution investigation of critical or charging specimens.
In the LVSEM, both SEs and BSEs travel in substantially same directions. Therefore the signal electrons collected by an electron detector will be the combination of SEs and BSEs, and the image may comprise topography contrast, material contrast and voltage contrast due to conventional electron detectors have very low sensitivities to electron energies. To clearly show some interested features, the image in an application may be required to comprise only one kind of the contrasts. Accordingly, the detection of the signal electrons (SEs and BSEs) is better sensitive to electron energies; i.e. it is desired to use an energy-discrimination detection which can select signal electrons in terms of energies thereof. Typically, the energy-discrimination detection is realized by making the electrons pass through an energy filter before being collected by a conventional detector.
The intensity distributions of SEs and BSEs in the beam of secondary emission depend on initial kinetic energies and emission angles thereof, and therefore the filtering function of the energy filter is preferred to be energy-depending other than energy-angle-depending. The energy-depending filtering is only sensitive to electron energies, and the energy-angle-depending filtering is sensitive to electron emission angles as well as electron energies. For the secondary emission by a low-energy PE beam, the angular distributions of SEs and BSEs respectively conform Lambert's law (proportional to cos φ, where φ is emission angle relative to the surface normal of the specimen). The large spread in emission angles makes it difficult to realize a pure energy-depending filtering, and the real filtering function depends on electron emission angle more or less. In addition, because of the deflection effect of the deflector(s) and the geometric magnification effect of the objective lens on the trajectories of the SEs and BSEs, the incident situations of SEs and BSEs, when entering the energy filter, change with the original locations on the field of view (FOV) of the specimen surface. For example, the beam of SEs from the FOV center is incident onto the entrance of the energy filter along the optical axis thereof, while the beam of SEs from an edge point of the FOV has 2° incident angle and 3 mm off-axis shift (both relative to the optical axis). Consequently, the filtering function of the energy filter actually also depends on the original positions of electrons on the FOV more or less, thereby being position-depending filtering to a certain degree.
The available energy filters can be classified into two types, dispersion (axial or radial) type and reflection type. An energy filter of axial or radial dispersion type uses a dispersive element to make electrons with different energies generate different displacements in axial or radial direction correspondingly, while an energy filter of reflection type uses a potential barrier to reflect back the electrons with initial kinetic energies not higher than a specific value so as to prevent them from passing through. The dispersive element can be a lens (U.S. Pat. Nos. 7,544,937, 7,683,317) or a deflector (U.S. Pat. No. 7,276,694), and the potential barrier can be an equipotential formed by a hollow electrode (U.S. Pat. No. 7,335,894) or a grid electrode (U.S. Pat. Nos. 7,141,791, 7,544,937, 7,683,317, 7,714,287, 8,203,119). Energy filters of axial dispersion and reflection types are well used individually or in combination in the LVSEM due to being compact and simple in configuration.
Accordingly, the filtering function of the energy filter can be evaluated by energy-discrimination power at the FOV center and uniformity of energy-discrimination powers over the entire FOV within the required range (such as 0.2 eV˜5 keV for a LVSEM) of the landing energy of the PE beam. The energy-discrimination power for a point in the FOV is the variation of energy thresholds with respect to the emission angle spread, while the uniformity of energy-discrimination powers over the entire FOV is the variation of the energy thresholds of the chief rays (with 0° emission angles) coming from the entire FOV. For the energy filter of reflection type such as the energy filter 3 in front of the detector 7 in FIG. 1A, if the energy thresholds for 0° emission angle (the chief ray 2c) and 45° emission angle (the margin ray 2m) of the SEs from the center point BO of the FOV on the surface of the specimen 4 are 2 eV and 2.1 eV respectively, the energy-discrimination power is 0.1 eV with respect to 45° emission angle spread for the FOV center. If the energy thresholds of the chief rays over the entire FOV of 50 um×50 um square are within the range of 2 eV˜3 eV, the uniformity of the energy-discrimination powers over the entire FOV is equal to the range, i.e., 1 eV. The lower the variation of the energy thresholds for the FOV center is, the fine the energy-discrimination power will be; while the lower the range of the energy thresholds of the chief rays over the FOV covers, the higher the uniformity of the energy-discrimination powers will become.
Energy filters of reflection type can be placed near the specimen surface or the detector, as shown in FIGS. 1B and 1C. In both FIGS. 1B and 1C, the PE beam 1 is finally focused by the objective lens 5 and lands on the specimen 4. The electron beam 2 of secondary emission is emitted from where the specimen 4 is excited by the PE beam 1, which comprises SEs such as the SEs 2_1 and BSEs such as the BSEs 2_2. In FIG. 1B, the hollow electrode 6 is negatively biased with respect to the specimen 4 to form a potential barrier PB1 therebetween, and the SEs 2_1 with initial kinetic energies not higher than the energy thresholds with respect to the potential barrier PB1 are therefore reflected back to the specimen 4 and the other SEs and BSEs can pass the potential barrier and can be detected by the detector 7. This method can not be used to an application requiring a strong extraction field on the specimen surface to make more electrons escape from specimen surface or get high resolution. In FIG. 1C, the grid electrode 8 is negatively biased with respect to the specimen 4 and becomes a potential barrier PB2 itself, and SEs 2_1 with energies not higher than the energy thresholds with respect to the potential barrier PB2 are reflected back.
For an energy filter of reflection type, if the electrons which come from the FOV center with same energies and different emission angles can approach the potential barrier with substantially equal angles of incidence (relative to the correspondingly local normal of the potential barrier) such as shown in FIGS. 2A and 2B, the energy-discrimination power will become finer. Meanwhile, if the electrons of the chief rays which come from the entire FOV with same energies can also approach the potential barrier with substantially equal angles of incidence, the uniformity of the energy-discrimination powers over the FOV will become higher. If the potential barrier has a shape of sphere (corresponding to a focused beam) or plane (corresponding to a parallel beam), it is possible to meet the foregoing requirements. In the light of the potential barrier shape, the potential barrier formed by a hollow electrode (such as in U.S. Pat. No. 7,335,894) has a shape of hyperboloid and therefore is not advantageous than the potential barrier formed by a flat grid electrode in getting a fine energy-discrimination power. Furthermore, although the hollow electrode incurs no electron loss due to lack of the electrons hitting on the wires of the grid electrode, the dramatically deteriorated energy-discrimination power due to the strong energy-angle-depending filtering and position-depending filtering puts a limitation onto the acceptable emission angle spread and the acceptable size of the FOV, thereby limiting the intensity of the image signal and the throughput of the application.
Among the foregoing patents using a grid electrode to form a potential barrier, some (U.S. Pat. Nos. 7,683,317 and 8,203,119) assume the secondary emission beam is parallel when entering the energy filter and thereafter attempt to keep it parallel and normally incident onto the potential barrier, some (U.S. Pat. Nos. 7,141,791 and 7,714,287) adjust the incident direction of the secondary emission beam with respect to the position thereof in the FOV before entering the energy filter, and some (U.S. Pat. No. 7,544,937) makes the grid electrode with a special shape to fit the secondary emission beam. The foregoing cases either can only work well within a small emission angle spread and a small FOV, or needs at least an additional element for specially adjusting the secondary emission beam before entering the energy filter or a complex grid. To reduce the emission angle spread to fit the energy filter, a lot of electrons with large emission angles in the secondary emission beam have to be cut off, thereby reducing the number of the detected signal electrons. To get a stronger image signal, the scanning speed has to be slow down to increase the integration time of each pixel in the image. The small FOV corresponds to a low throughput of observation because more moving steps are required to observe a large area on the specimen surface. The additional space is thus needed to accommodate the additional adjusting element(s), thereby making the entire apparatus bulky. Apparently, in the available energy filters of reflection type, there is no means for directly improving the incident situation of a secondary emission beam on the potential barrier if the secondary emission beam is not parallel when entering the energy filter.
Accordingly, an energy filter for energy-discrimination detection in a LVSEM, which can provide a fine energy-discrimination power within a large emission angle spread and a high uniformity of energy-discrimination power over a large FOV, is needed. Such an energy filter will be more advantageous to improve the image contrast than the prior of art.